A seismic survey represents an attempt to image or map the subsurface of the earth by sending sound energy down into the ground and recording the “echoes” that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, or air guns in marine environments. During a seismic survey, the energy source is placed at various locations near the surface of the earth above a geologic structure of interest. Each time the source is activated, it generates a seismic signal that travels downward through the earth. “Echoes” of that signal are then recorded at a great many locations on the surface. Multiple source/recording combinations are then combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2-D) seismic survey, the recording locations are generally laid out along a single line, whereas in a three dimensional (3-D) survey the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2-D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3-D survey produces a data “cube” or volume that is, at least conceptually, a 3-D picture of the subsurface that lies beneath the survey area. In reality, though, both 2-D and 3-D surveys interrogate some volume of earth lying beneath the area covered by the survey. Finally, a 4-D (or time-lapse) survey is one that is recorded over the same area at two or more different times. Obviously, if successive images of the subsurface are compared any changes that are observed (assuming differences in the source signature, receivers, recorders, ambient noise conditions, etc., are accounted for) will be attributable to changes in the subsurface.
A seismic survey is composed of a very large number of individual seismic recordings or traces. The digital samples in seismic data traces are usually acquired at 0.002 second (2 millisecond or “ms”) intervals, although 4 millisecond and 1 millisecond sampling intervals are also common. Typical trace lengths when conventional impulsive sources are used are 5-16 seconds, which corresponds to 2500-8000 samples at a 2-millisecond interval. If a non-impulsive source is used, the extended activation time of the source needs to be accommodated for, so the trace lengths will generally be longer. Conventionally each trace records one seismic source activation, so there is one trace for each live source location-receiver activation. In some instances, multiple physical sources might be activated simultaneously but the composite source signal will be referred to as a “source” herein, whether generated by one or many physical sources.
In a typical 2-D survey, there will usually be several tens of thousands of traces, whereas in a 3-D survey the number of individual traces may run into the multiple millions of traces.
Although a number of seismic sources are available, controllable sources have been used for many years to obtain land and marine seismic data for use in exploration, reservoir evaluation, etc. For purposes of the instant disclosure, the term “controllable source” will be used to refer to an acoustic seismic source that radiates sound as a swept-frequency signal, whose profile of frequency versus time after the start of the sweep is controllable and continuous, and whose physical limitations impose a limit on the amplitude of its output which will normally vary with frequency.
Controllable sources may be used in circumstances where it is desired to explore an environment via its acoustic response. For example, in exploration seismology signals are radiated into the ground and the echoes that return to the surface are recorded and used to investigate the geology of the subsurface to identify possible locations of oil and gas reservoirs. Controllable sources include, by way of example only, vibroseis sources on land and at sea, marine resonators, etc. General information related to marine resonators may be found in, for example, U.S. patent application Ser. Nos. 12/980,527 and 12/995,763, the disclosures of which are incorporated herein by reference as if fully set out at this point.
In such circumstances it is desirable to control the shape of the spectrum of the acoustic signal transmitted by the source. It is typically also desirable to maximize the source's acoustic power output, which will be limited by its physical capabilities. The power output can be maximized, of course, by simply running the source at the maximum output level it is capable of. In general, though, this level will vary strongly with frequency, so that if the device is swept in frequency at a constant rate it will produce a spectrum whose shape, or variation in relative magnitude with frequency (hereinafter its “frequency profile”) is dictated by the source's physical limitations and may not be optimal for the application. For example, the output of a piston source at low frequencies is limited by its piston stroke and is inversely proportional to frequency squared, whereas for the purposes of seismic exploration it is typically desirable to have a flat or approximately flat spectrum. To the extent the spectrum is not flat, distortions appear in the subsurface images that may be difficult or impossible to remove.
Thus, what is needed is a way to generate a seismic signal with a controllable source such that the said signal has frequency properties that have been chosen to yield better images of the subsurface, and to use the controllable source in a maximally efficient way while doing so.
It should be noted that prior art attempts to do this have focused solely on the displacement of the piston as a limiting parameter at low frequencies, and fall back on traditional linear sweeps for higher frequencies, which approach has proven to be unsatisfactory in many cases.
As is well known in the seismic acquisition and processing arts, there has been a need for a system and method that provides a better way to acquire broadband (e.g., about 1-80 Hz) seismic data. Accordingly, it should now be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for a method of seismic data processing that would address and solve the above-described problems.
Before proceeding to a description, however, it should be noted and remembered that the disclosure which follows, together with the accompanying drawings, should not be construed as limiting the teachings of this document to the examples (or embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.